1. Field of the Invention
The present invention relates to a DC power transmission system interconnecting two AC systems.
2. Description of the Prior Art
In a DC power transmission system, a first external commutated converter operating as a rectifier and a second external commutated converter operating as an inverter are connected with each other at their DC sides via DC reactors and a DC transmission line. Each converter is connected at its AC side to an AC system via a transformer and a circuit breaker.
Control system controlling the converters has an interchange power control system at the first converter and a margin angle control system at the second converter.
The interchange power control system controls the DC output current of the first converter operating as a rectifier to make the active interchange power measured at the AC system of the first converter have a set value. On the other hand, the margin angle control system controls the lead angle of the second converter to have a margin angle sufficient for preventing commutation of the second converter from failing. A margin angle controller is well known as described, for example, in Japanese Patent Publication No. 46956/1983.
The margin angle control system maintains a proper margin angle of the second converter and determines a DC voltage of the DC transmission line, while the interchange power control system controls the DC current. Consequently, a stable operation of the DC power transmission system can continue. Such a control system is a known technique as described, for example, in Japanese Patent Application Laid-Open No. 107443/1975.
An external commutated converter used for interconnection between two AC systems and operating as an inverter is a lagging power factor load as seen from the AC system so that it receives necessary reactive power from the AC system. Conventionally, in such a case, technical concept for controlling the reactive power of the converter has not been employed.